A bug to move slowly if necessary (2,1,5)
I believe the answer is:
at a pinch
'if necessary' is the definition.
(at a pinch means if required)
'a bug to move slowly' is the wordplay.
'bug' becomes 'tap' (synonyms).
'to move slowly' becomes 'inch' (to inch along is to move slowly).
'a'+'tap'+'inch'='AT A PINCH'
(Other definitions for at a pinch that I've seen before include "under pressure" , "In time of difficulty or necessity - nip it" , "in case of emergency" , "in a crisis?" , "If absolutely necessary" .)